This project is directed toward the identification and study of thyroid hormone dependent cells in the developing central nervous system of mammals and lower vertebrates. Histochemical, histological, autoradiographic, and electron microscopic techniques will be employed to identify, in thryroprivic, hypothyroid and hyperthyroid animals, those neurons and neuroglial elements which react to alterations in thyroidal state and to characterize the nature of these observed morphological and functional changes.